Various devices have been developed for sensing and measuring the concentrations of different gases at man-made or natural locations, such as oil wells, pipelines, mines, manufacturing plants, refineries, and the like. Monitoring for the presence and concentration of gases may be used for various applications, such as to ensure that toxic gases (CO, H2S, etc.) are not present in significant concentrations, to ensure that explosive gases (CH4, H2, etc.) are below respective explosive limits, to identify the gases in a mixture (for custody transfer, heat content, etc.), or for various other reasons. Spectroscopy may be used to provide highly sensitive and selective sensors because each gas exhibits a unique spectroscopic fingerprint, such that gases absorb and emit light energy at specific wavelengths. Gases are relatively transparent, however, so the absorption line strength of a gas may be relatively small and hard to detect.
To accommodate for the small absorption line strength, light used in spectroscopy is required to pass through long path lengths in the gas in order to establish sufficient sensitivity for spectroscopic sensor to provide a measurement of a concentration of a gas of interest in a test sample, for example. For example, a light source of the spectroscopic sensor may be separated from a detector of the spectroscopic sensor by a distance of one kilometer or more to achieve a necessary path length, but such distances are not practical in most applications.
Other known types of spectroscopic sensors define an optical cavity with two mirrors and are referred to as optical cavity sensors. The gas is contained within the optical cavity, and the light is reflected between the two mirrors multiple times before being detected. While this technique allows for a manageable device size, it is problematic due to the need to maintain very exacting alignment of the mirrors. Variations in conditions, such as temperature changes, vibration, humidity, or the like, may misalign the mirrors or otherwise interfere with the sensitivity and/or accuracy of these optical cavity sensors. Therefore, this technique is generally not used for remote, unattended measurements in various field environments, such as an oil or gas well pad, a pipeline, a mine, or the like. Moreover, optical cavity sensors are generally quite expensive.